The standard for optical transport of telecommunications in North America is Synchronous Optical Network or SONET and in other parts of the world there is a variant referred to as Synchronous Digital Hierarchy or SDH. The SONET and SDH standards specify various protection schemes such as line protection, ring protection, and path protection. Conventionally, a line comprises one or more network sections or spans between network elements. In SONET, a path is a logical connection between a point where a Synchronous Transport Signal (STS) or a Virtual Tributary (VT) (equivalent terms in SDH are Access Unit (AU) and Tributary Unit (TU), respectively) is multiplexed into the transport network and the point where the signal is demultiplexed.
Line, ring, and path protection schemes depend upon the various transport network architectures in which the schemes are operating, such as linear networks and ring networks (e.g., virtual line-switched ring or VLSR, unidirectional path-switched ring or UPSR, two- and four-fiber bi-directional line switched rings or BLSR). Whereas SONET and SDH support such protection schemes, these standards refer to them by different names. For example, in SONET, one type of line protection is called 1+1; in SDH the same type of line protection is referred to as MSP. One example of path switching in SONET is UPSR; in SDH, SNCP. As another example, one form of ring protection in SONET is BLSR; for SDH, MSSPring. Additional types of protection schemes include, for example, 1:N linear protection switching.
Corresponding SONET and SDH protection schemes specify different requirements for triggering their protection operation. In general, when shipped from the factory, network elements to be deployed in a synchronous data communications network are configured to support protection switching according to one optical transport standard or the other. That is, a network element is configured to provide protection switching either for SONET or for SDH. Therefore, to support both SONET and SDH, manufacturers need to produce two type of network elements, which results in increased costs to the manufacturer.